(1) Field of the Invention
The present invention relates to novel chemical compounds, and more specifically, it relates to a new class of compounds which, in the presence of base, exhibit color upon the application of heat and a reversion to the original substantially colorless state upon cooling.
(2) Description of the Related Art
A number of compounds which undergo a change in color upon the application of heat and a reversion to their original color upon cooling are known in the art. This reversible, thermal triggering of a color change is called thermochromism. Examples of such compounds include spiropyrans, ethylenic compounds, e.g., dixanthylene, bianthrone, and xanthylideneanthrone, and disulfides, e.g., diphenyldisulfide and .beta.-dinaphthyldisulfide. For a review of thermochromism and thermochromic compounds reference can be made to, for example, Sabongi, G. J., Chemical Triggering, Reactions of Potential Utility in Industrial Processes, Ch. 5, pp. 240 to 278 (1987); and Day, J. H., Chem. Rev., 63, 65 (1963).
U.S Pat. No. 4,567,019 discloses 3,3'-bis(p-aminophenyl)phthalides which behave as thermochromes in the presence of certain organic acids. This thermochromic behavior is suspected to be the result of a temperature dependent hydrogen bonding equilibrium between one of the amine radicals of the phthalide and the --OH or --SH group of the acid.